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Propeller MRI In Chan Song, Ph.D. Seoul National University Hospital

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Contents: Propeller sequence (Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction) Motion artifact Theoretical basis Applications

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Motion Periodic: cardiac motion, respiration, blood flow Sporadic: irritable patients’ motion Translation, rotation, through-plane Artifact in MRI blurring and ghosting Cause Longer encoding step

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Scan time= TR x matrix x Average Long scan time

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MR image reconstruction under the assumption of object’s motion-free condition during whole k space coverage

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Motion artifacts -Most ubiquitous and noticeable artifacts in MRI due to voluntary and involuntary movement, and flow (blood, CSF) -Mostly occur along the phase encode direction, since adjacent lines of phase-encoded protons are separated by a TR interval that can last 3,000 msec or longer -Slight motion can cause a change in the recorded phase variation across the FOV throughout the MR acquisition sequence

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Motion artifact: ghost and blurring

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Solution for motion compensation -Navigator echo usage to estimate the motion or motion related phase from extra collected data -Cardiac and respiratory gating -Respiratory ordering of the phase encoding projections based on location in respiratory cycle -Signal averaging to reduce artifacts of random motion -Short TE spin echo sequences (limited to spin density, T1-weighted scans). Long TE scans (T2 weighting) are more susceptible to motion

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Motion (abrupt) phase error position error Solution Phase information Navigation Motion correction by phase information

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Key ideas in propeller sequence K space: partial covering for whole image Motion detection: blade usage Correction: FFT properties’ usage

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Diagram of the PROPELLER collection reconstruction process for motion corrected MRI.

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Rectangular filling kx ky Data acquisition Propeller filling

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Phase Correct Redundant data must agree, remove phase from each blade image Imperfect gradient balancing, Eddy current effect: echo center shift

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James G. Pipe

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Windowing BeforeAfter Phase correction

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Bulk Transformation Correction Fourier transform correspondence Image space k space Translation Phase roll Rotation Rotation Separate estimation of rotation and translation

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rotate image rotate data Fourier Transform Properties

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Reference (only inner circle) Magnitude of the average of strips Rotation (only inner circle) Correlation Rotation correction (magnitude image)

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Blade by blade operation Rotation at maximum correlation Correction

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Fourier Transform Properties shift image phase roll across data b is blade image, r is reference image

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max at x

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Translation Complex average k-space data Reference (only inner circle) Complex of the average of strips Inverse FT (maximum) Multiplication

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Blade by blade operation Translation at maximum correlation Correction

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Blade Correlation throw out bad – or difficult to interpolate - data

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Through-plane motion :low weighting coeff.

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Reconstruction (FFT) non-Cartesian sampling requires gridding convolution Kx Ky

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w/motion correction

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no correction correlation correction only motion correction only full corrections

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T2-FSE T2-PropellerT2-Propeller(corrected) Artifact reduction due to head motion

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DWI-EPI B=1000s/mm2 DWI-Propeller (FSE) James G Pipe, 2002

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DWI (b=700s/mm2) a. EPI (TR/TE/avg=2700/113/15) b. Propeller EPI (TR/TE/blade=1600/70/26) Wang FN, 2005

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Useful application in propeller sequence Motion- or Bo-inhomogeneities – insensitive Irritable patient Diffusion weighted image Limitations in propeller sequence Redundant acquisition Long scan time: High SAR: problem in higher field MR system Solutions Undersampling (Konstantinos Arfanakis, 2005) Parallel imaging Turbopropeller (James G Pipe, 2006) Propeller EPI

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Propeller sequence Low sensitivity to image artifacts, Bo inhomogeneity and motion T2-, Diffusion-weighted images (High SNR, low geometric distortion, low SAR)

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References 1. Pipe J, MRM 42(5): , Pipe J, et al., MRM 47(1): 42-53, Wu Y, Field AS, Alexander AL. ISMRM, Toronto, Canada, Roberts TP, Haider M. ISMRM, Kyoto, Japan, Sussman MS, White LM, Roberts TP. ISMRM, Kyoto, Japan, Pipe J and Zwart N. Magn Reson Med 55:380–385, Cheryaukaa AB, et al. Magnetic Resonance Imaging 22: , 2004

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